The surface tension of a liquid can be measured for example by measuring the force applied to a probe placed in the interface between the said liquid and a gas, typically air. Such a probe can be in the form of a thin platinum plate such as a Wilhelmy plate, which is placed in the said interface. An alternative construction for the probe is in the form of a small diameter metal wire, for example a du Nouy ring.
A change in the surface tension is evidenced as a change in the amount of liquid adhered to the probe. When the surface tension of the liquid decreases, the amount adhered to the probe decreases linearly, and vice versa. The liquid adhered exerts a vertical force on the probe, which can be detected using a microbalance. Methods for measuring the surface tension at an air/water interface are described in WO 02/055996, entitled “Method for measuring the surface tension of an aqueous solution” and in WO 2004/025277, entitled “Method for surface tension measurement” which are included herein for reference.
In addition to measuring the surface tension at an interface between a liquid and a gas, such as air, there is a need for measuring the surface tension, and changes in the surface tension, at an interface between liquids that are essentially immiscible. Such information can be of importance for example in order to establish the emulsification properties of surfactants and detergents. When developing detergents it is essential to know how rapidly or effectively the detergent will emulsify or dissolve fat or oil, i.e. how effectively the fat and oil-like substances will be removed, for example during a washing operation. Such effects can be measured by measuring changes taking place in the surface tension at an interface between two liquids, such as an oil and an aqueous solution containing said detergent.
A problem has, however, been associated with the proper formation of an interface between the liquids which will allow for a rapid, correct and sensitive measurement of the surface tension and any changes therein. It would also be desirable to develop a system which would allow for rapid and efficient interface formation and surface tension measurement on a large-scale or screening basis, to be used for example in product development such as for the assessment of the surface active properties of various products, typically dissolved in one of the liquids being assessed. The said system could also be a useful tool in research relating to the study of kinetics between liquids.